Water used in circulating water systems (such as, for example, swimming pools or other recreational water systems) is commonly treated with a halogenated sanitizer to kill and control the growth of bacteria, viruses, algae, etc. A minimum level of sanitizer residual is commonly maintained in such systems to kill potentially pathogenic bacteria that may be introduced into the system. For example, sanitizer manufacturers and the federal government require within approved label directions for all chlorine and bromine based sanitizers that a halogen residual be maintained and be measured by an appropriate chemical method for “chlorine” or “bromine.”
To better operate pools and spas, automatic chemical feeders and automatic controllers may be used. The controllers are designed to feed chemicals either on a timer basis, or based on the chemical properties of the water. The chemical properties of the water are measured by various methods. The most accurate and precise methods to monitor sanitizer residuals use either an automated titration or an amperometric probe. Controllers that use these methods are more commonly used in drinking water and some pool and spa markets.
Oxidative Reduction Potential (ORP), which is a relatively inexpensive technique, has gained some acceptance in the United States pool and spa market, but has several drawbacks as a method to monitor and control the sanitizer levels. For example, ORP does not measure the “chlorine” or “bromine” sanitizer level directly, it measures the oxidative state of the water.
ORP readings are subject to interference from non-biocidal oxidizers and other chemicals commonly found in treated water. In that regard, ORP probe fouling often causes ORP readings to decrease even when the true ORP—or the true oxidative state—remains unchanged. Similarly, the ORP reading can decrease and the sanitizer levels would remain the same. Moreover, in treated water that uses isocyanurate such as trichloro-s-triazinetrione (trichlorisocyanuric acid or TCCA) or dichlorisocyanuic acid or dimethyl or methylethylhydantoins that have N,N-dibromo-, N,N-dichloro-, or N-N-bromochloro substituents, the problem seems to be exacerbated.
For example, in water that is treated with isocyanurate sanitizers it is not uncommon for the ORP reading to drop by 50-100 mV within a few days or a few weeks even if the true sanitizer level remains unchanged. The result is that a controller that is operating at a predetermined ORP set point (e.g. 650 mV) will continue to feed sanitizer causing the chlorine or bromine residual to exceed the range typically maintained.
In view of the above, it is well established that ORP probes must be cleaned to obtain a fast response and an accurate reading. However, to manually clean ORP probe surfaces is not practical in many applications since a knowledgeable technician is needed and strong acids are commonly used.
Accordingly, several different types of automatic cleaning systems have been developed in an attempt to reduce the cleaning requirements for ORP probes. These ORP probe cleaning systems have been used in certain large applications; but they are complicated, expensive and require a motor, an ultrasonic device, or cleaning supplies like compressed air, high-pressure water, or concentrated acids therefore unsuitable for smaller water treatment applications.
Regardless of the cleaning method, it is widely taught and practiced that the ORP probe metal surfaces should be allowed to stabilize at which time an oxidative film is formed on the metal surface. This stabilization time often takes several hours. As a result, it is commonly believed that the ORP metal surface should not be cleaned constantly because a stable ORP surface would not be achieved.
In addition to the above, it is also known that probe fouling is a significant problem for pH probes. As with ORP probes, the sensing surface of a pH probe must remain clean for accurate measurement. When the sensing surface is not clean, slow responses and inaccurate calibration may result.
A need therefore exists for a simple, cost effective way to keep ORP probes and/or pH probes clean when used to monitor recirculating water systems. The present invention addresses that need.